Project Abstract/Summary The brains of mammals contain an extraordinarily large number of neurons whose activity and interconnections determine the function of circuits that monitor our sensory environment, dictate our motor choices, form memories, and guide all behavior. However, we do not understand how the activity of these circuits governs brain activity. A fundamental limitation has been the inability to monitor and control the activity of a genetically and anatomically known brain cells. In order to gain insight into how circuit computations are carried out and subsequently control behavior, we will develop novel technologies. The first is a novel way to deliver light into the brain and collect light from the brain in a controlled manner in order to be able to perturb and probe the activity of neurons with high precision. The second is a new class of integrated optical and electrical devices that permit access to the same set of neurons by both modalities simultaneously. The third is a novel way to change the genetic composition of neurons that allows the recording device itself to induce the genetic change.